Perform periodic screening of relatives of patients with medullary carcinoma of the thyroid using calcitonin radioimmunoassay. Provide accurate, sensitive, and specific measurements of immunoreactive calcitonin on samples of plasma or tissue from patients suspected of having medullary carcinoma of the thyroid. Radioimmunoassays will be performed as needed to screen subjects for the presence of disease, or to follow the clinical progress of disease treated by surgery or chemotherapy. The primary responsibility will be in the measurement of samples obtained from patients followed in our MEN-II kindreds. Assay samples sent by other physicians in the southeastern United States if these physicians can provide basic clinical information on each patient. The information thus obtained may result in the discovery of new cases which might enter the study.